Arjan Tabak

Sinking city, rising sea levels

Over the past 20 years, Ho Chi Minh’s population has doubled to 8.8 million, making the city one of Asia’s fastest-growing economic centers as it is expected to exceed 12 million by 2035. Facilitating this population will present major challenges, such as water supply and climate adaptation. Ho Chi Minh City’s geographic location – around sea level, close to the sea, and in a delta – makes this area very vulnerable to land subsidence. Soil subsidence will lead to damage to urban infrastructure (roads, sewers, subway, bridges, etc.) and water works (dikes and locks), as well as create greater flood risks. Already, some parts of the city flood regularly under the influence of the tides.

The effects that floods can have in Ho Chi Minh City (source: Geomatics Center, Dinh et al, 2016)

1. Relevance

Effective measures can only be taken if the cause and extent of land subsidence is understood. To this end, Sensar collaborated with local parties (Geomatics Center and the Center of Water Management and Climate Change (VNU-WACC)) to map land subsidence in Ho Chi Minh City using ‘state-of-the-art’ InSAR data. Using Copernicus data, the elevation change was mapped over a 2-year period (2017-2019) with weekly recordings. The result is available in a 4D visualization created in collaboration with our partner Klaas Nienhuis.

2. The 4D visualization

The interactive demo clearly shows the ground motion over time. The movement is relative to the first observation and to a stable reference point. For the visualization, only ground level measurements were used (without buildings). Most subsidence occurs at the edges of the city, because the old city is built on a stable layer of sand and sandstone, but is surrounded by weaker soils (mainly clay and loam). The rapid expansion of Ho Chi Minh City is leading to increasing development of the outlying areas and the need to pump increasing amounts of groundwater to supply the growing population, industry, and agriculture. The combination of the additional strain on these sagging soils and the sinking water table leads to sharp subsidence in the outer edges of the city, at rates of up to 80 mm per year. Combined with a sea level rise of about 3 mm per year, the region faces a major challenge.

3. Understanding future flood risks.

Our partners at the WACC and WaterLand Experts have used land subsidence data to estimate future flood risks. These floods are caused by water flow from rivers, both downstream and upstream (tidal action), and heavy rainfall. Based on the land subsidence values from the satellite data, projections were made of the future elevation of the area. These projections show that some areas, such as Districts 8 and 12, are expected to fall below sea level. As a result, these areas will experience major flooding over 2 meters deep. Other areas, such as District 7 and Nha Be, will experience flooding of between 0.4 and 0.8 meters. Until previously, insights at this level of detail were not available: the large scale and quality of satellite-based subsidence measurements now make this possible. With these projections, data-driven decisions can be made to mitigate future land subsidence and protect vulnerable and critical areas and infrastructure.

Projected flood risks in 2050. The future elevation of the area was calculated for this model, based on land subsidence as measured by Sensar.